(a) Field of the Invention
This invention relates to an improvement in the electrowinning of zinc from acid zinc sulfate solution.
(b) Description of the Prior Art
In the process for the recovery of zinc from acid zinc sulfate solution, zinc calcine or zinc concentrate is leached at elevated temperature and at atmospheric or elevated pressure with return sulfuric acid or spent electrolyte from the electrowinning process. The leach solution is neutralized and subjected to a number of purification steps using manganese dioxide and zinc dust, and the purified solution is subjected to electrolysis for the electrowinning of zinc. Electrolysis is carried out in electrolytic cells, using aluminum cathodes and lead-silver anodes. Addition agents such as glue and strontium or barium carbonate are used to obtain even deposits of substantially pure zinc. A zinc deposition period of between 24 and 72 hours is normally used, after which cathodes are removed from cells for the recovery of deposited zinc and are replaced with fresh cathodes. Anodes are removed periodically for cleaning and are subsequently returned to the cells. The time between cleanings of anodes varies but is usually between three and ten weeks. The electrolytic cells are periodically shut down and cleaned to remove cell mud. Electrolyte overflows from the cells and is usually recirculated in a closed circuit. If necessary, the recirculation circuit comprises a heat exchanger to cool the circulating electrolyte. A portion of the recirculating electrolyte is normally bled from the system and returned as return acid or spent electrolyte to the leaching operation of calcine or concentrate.
The process, as generally described above, is disclosed in Zinc, C. H. Mathewson, an A.C.S. Monograph, Reinhold Publishing Corporation, New York, 1959, pages 174,225, and is used with minor variations in most electrolytic zinc plants in the world. The circulation of electrolyte, specifically, is disclosed in U.S. Pat. No. 1,282,521 according to which metals are recovered in a process consisting of leaching metalliferous material with spent electrolyte, electrolyzing the leach solution, circulating electrolyte through the electrolytic cells and withdrawing a portion of the circulating electrolyte for return to the leach.
During electrolysis of purified acid zinc sulfate solution, a sludge forms which mainly consists of manganese dioxide, precipitated compounds such as barium or strontium sulfate and calcium sulfate, and precipitated anode corrosion products, such as lead sulfate and lead oxides. A major portion of the formed sludge settles in the bottom of the cells, while a minor portion remains suspended in the circulating electrolyte. Only a part of this minor portion is removed with electrolyte that is returned to the leaching operation and the remainder is recirculated. According to U.S. Pat. No. 2,072,811, there is disclosed a method for electrolyzing a sludge-free zinc-containing electrolyte comprising passing electrolyte through an electrolytic cell with sufficient velocity to carry formed sludge from the cell and contacting the electrolyte-containing sludge with zinc-bearing material to dissolve zinc in electrolyte. The method of sludge removal according to this last named patent is quite impractical, because the flow velocities that are required to remove all sludge from the cell would be undesirably high. Consequently, in most commercial zinc electrowinning plants, sludge formed during electrolysis and not adhering to the anodes settles in the electrolytic cells. This necessitates periodic cleaning of cells.
We have observed, however, that the formation of a major portion of sludge occurs in cells that contain new anodes and/or anodes that have been recently cleaned. We have also found that electrolyte overflowing from these cells contains a quantity of sludge particles that is considerably higher than electrolyte overflowing from cells that contain anodes which have been in operation for at least one day. Moreover, we have observed that the rate of corrosion of anodes is highest for new and recently cleaned anodes and that the corrosion rate decreases sharply after the new and recently cleaned anodes have been in use for about one day. As a result, the consumption of addition agents such as barium or strontium carbonate to control the lead content in electrolyte and deposited zinc is highest in cells containing new and recently cleaned anodes.